BackgroundPhotosynthetic organisms utilize carotenoids for photoprotection as well as light harvesting. Our previous study revealed that high-intensity light increases the expression of the gene for phytoene synthase EgcrtB in Euglena gracilis a unicellular phytoflagellate, the encoded enzyme catalyzes the first committed step of the carotenoid biosynthesis pathway. To examine carotenoid synthesis of E. gracilis in response to light stress, we analyzed carotenoid species and content in cells grown under various light intensities. In addition, we investigated the effect of suppressing EgcrtB with RNA interference RNAi on growth and carotenoid content.

ResultsAfter cultivation for 7 days under continuous light at 920 μmol m s, β-carotene, diadinoxanthin Ddx, and diatoxanthin Dtx content in cells was significantly increased compared with standard light intensity 55 μmol m s. The high-intensity light 920 μmol m s increased the pool size of diadinoxanthin cycle pigments i.e., Ddx + Dtx by 1.2-fold and the Dtx-Ddx ratio from 0.05 control to 0.09. In contrast, the higher-intensity light treatment caused a 58% decrease in chlorophyll a + b content and diminished the number of thylakoid membranes in chloroplasts by approximately half compared with control cells, suggesting that the high-intensity light-induced accumulation of carotenoids is associated with an increase in both the number and size of lipid globules in chloroplasts and the cytoplasm. Transient suppression of EgcrtB in this alga by RNAi resulted in significant decreases in cell number, chlorophyll, and total major carotenoid content by 82, 82 and 86%, respectively, relative to non-electroporated cells. Furthermore, suppression of EgcrtB decreased the number of chloroplasts and thylakoid membranes and increased the Dtx-Ddx ratio by 1.6-fold under continuous illumination even at the standard light intensity, indicating that blocking carotenoid synthesis increased the susceptibility of cells to light stress.

ConclusionsOur results indicate that suppression of EgcrtB causes a significant decrease in carotenoid and chlorophyll content in E. gracilis accompanied by changes in intracellular structures, suggesting that Dtx de-epoxidized form of diadinoxanthin cycle pigments contributes to photoprotection of this alga during the long-term acclimation to light-induced stress.